A depression before the bump in the highest energy cosmic ray spectrum

We re-examine the interaction of ultra high energy nuclei with the microwave background radiation. We find that the giant dipole resonance leaves a new signature in the differential energy spectrum of iron sources located around 3 Mpc: A depression before the bump which is followed by the expected cutoff. This feature could be related to the apparent gap in the existing data and thus support ``bottom up'' models for the origin of the highest energy cosmic rays